Motion Detector Light Fading

[Old Bill]

Years back I installed a motion detection light on the front of my garage. I changed the bulb once I think. But am noticing now the light is not nearly as bright as it used to be. It happened suddenly. The bulb in use is either a tungsten or a halogen. I get confused. Its one of those tubular types with ceramic ends and you cannot touch the bulb with bare fingers. I think tungsten but maybe someone will correct me. Is it possible the bulb is wearing out? Never heard of that happening before - they usually just pop. I also noticed that the light is not always coming on when motion is detected.

 

[2ManyProjects]

Years back I installed a motion detection light on the front of my garage. I changed the bulb once I think. But am noticing now the light is not nearly as bright as it used to be. It happened suddenly. The bulb in use is either a tungsten or a halogen. I get confused. Its one of those tubular types with ceramic ends and you cannot touch the bulb with bare fingers. I think tungsten but maybe someone will correct me.

The correct answer is... (drum roll, please)... BOTH.

That bulb is commonly (and correctly) referred to as a "Halogen" type, due to the inert gas used to fill the envelope (glass tube). By comparison, a conventional incandescent lightbulb uses an evacuated envelope (i.e., a vacuum instead of an inert gas). Note that in BOTH cases, the filament inside the envelope is indeed made of Tungsten; so that term is not determinative.

Is it possible the bulb is wearing out?

Yes, especially if it has been operated off a dimmer. While I doubt that the case here, there COULD be reasons why, from the bulb's perspective, it has "seen" effectively the same thing.

In "normal" operation, a halogen bulb burns SO hot that it actually starts "boiling off" atoms from the filament material. These stray atoms float around in the inert gas, cool down, and are eventually attracted back to the filament via molecular attraction/bonding (I'm a little weak on the precise chemistry; but that's the gist of it).

However, when run off a dimmer control, the bulb burns significantly cooler; so this molecular redeposition process (which halogen bulbs need to provide their full expected lifespan) does not work as efficiently. Instead of landing back on the filament, those "stray" atoms of tungsten wind up coating the interior surface of the envelope. This does two things, both of them bad:

1. - That micro-thin film of tungsten on the envelope partially blocks the light from passing through the glass and out into the world. If you closely examine the bulb with the power off, you may discern a subtle darkening of the glass tube, as compared to a brand new bulb of the same type.

2. - The filament itself, being starved of the aforementioned material replenishment, gradually becomes thinner and thinner (it also becomes weaker; and when it gets TOO weak, it pops). This thinning down of the filament increases its resistance. More resistance == less current flow; and so the bulb burns dimmer for that reason, as well.

Now, as for WHY the bulb in that fixture might be showing signs of this phenomenon... I would look to the photocell/control circuit of the fixture it is installed in. If it uses a mechanical relay to switch the lamp on and off, the contacts on that relay could have become pitted and/or oxidized, which will of course limit current flow just like a dimmer would. If OTOH, it uses some sort of sold-state relay, it is (remotely) possible that it is no longer providing full voltage at its output, due to some sort of internal failure/degradation (I view this as unlikely, but not impossible).

I also noticed that the light is not always coming on when motion is detected.

Yet another reason to suspect the control circuit.

 

[Old Bill]

When I posted the questions I never ever thought I would receive such a comprehensive answer. I learned more in your reply than I could from a book. Very well explained and I thank you for that. I will replace the bulb today as part one of the "repair" and consider what to do about the switching mech. I do believe it is mechanical.
Thanks very much again.

 

[Kevin C]

Years back I installed a motion detection light on the front of my garage. I changed the bulb once I think. But am noticing now the light is not nearly as bright as it used to be. It happened suddenly. The bulb in use is either a tungsten or a halogen. I get confused. Its one of those tubular types with ceramic ends and you cannot touch the bulb with bare fingers. I think tungsten but maybe someone will correct me. Is it possible the bulb is wearing out? Never heard of that happening before - they usually just pop. I also noticed that the light is not always coming on when motion is detected.

A sudden change might be a bad connection from the bulb to the socket. Once they get a little loose or corroded they get hot and can dim the bulb or become intermittent.

Might try pulling the bulb and looking for heat damage on the contacts.

 

[wyliesdiesels]

The correct answer is... (drum roll, please)... BOTH.

That bulb is commonly (and correctly) referred to as a "Halogen" type, due to the inert gas used to fill the envelope (glass tube). By comparison, a conventional incandescent lightbulb uses an evacuated envelope (i.e., a vacuum instead of an inert gas). Note that in BOTH cases, the filament inside the envelope is indeed made of Tungsten; so that term is not determinative.



Yes, especially if it has been operated off a dimmer. While I doubt that the case here, there COULD be reasons why, from the bulb's perspective, it has "seen" effectively the same thing.

In "normal" operation, a halogen bulb burns SO hot that it actually starts "boiling off" atoms from the filament material. These stray atoms float around in the inert gas, cool down, and are eventually attracted back to the filament via molecular attraction/bonding (I'm a little weak on the precise chemistry; but that's the gist of it).

However, when run off a dimmer control, the bulb burns significantly cooler; so this molecular redeposition process (which halogen bulbs need to provide their full expected lifespan) does not work as efficiently. Instead of landing back on the filament, those "stray" atoms of tungsten wind up coating the interior surface of the envelope. This does two things, both of them bad:

1. - That micro-thin film of tungsten on the envelope partially blocks the light from passing through the glass and out into the world. If you closely examine the bulb with the power off, you may discern a subtle darkening of the glass tube, as compared to a brand new bulb of the same type.

2. - The filament itself, being starved of the aforementioned material replenishment, gradually becomes thinner and thinner (it also becomes weaker; and when it gets TOO weak, it pops). This thinning down of the filament increases its resistance. More resistance == less current flow; and so the bulb burns dimmer for that reason, as well.

Now, as for WHY the bulb in that fixture might be showing signs of this phenomenon... I would look to the photocell/control circuit of the fixture it is installed in. If it uses a mechanical relay to switch the lamp on and off, the contacts on that relay could have become pitted and/or oxidized, which will of course limit current flow just like a dimmer would. If OTOH, it uses some sort of sold-state relay, it is (remotely) possible that it is no longer providing full voltage at its output, due to some sort of internal failure/degradation (I view this as unlikely, but not impossible).



Yet another reason to suspect the control circuit.

Thx for the chemistry lesson.

I dont usually read your 'books' anymore(dont have time), but i DID read his one!!

 

[2ManyProjects]

When I posted the questions I never ever thought I would receive such a comprehensive answer. I learned more in your reply than I could from a book. Very well explained and I thank you for that.

You're welcome. But see below.

A sudden change might be a bad connection from the bulb to the socket. Once they get a little loose or corroded they get hot and can dim the bulb or become intermittent.

Might try pulling the bulb and looking for heat damage on the contacts.

I missed the "It happened suddenly" part first time through. That does indeed imply a bad/marginal connection SOMEWHERE in the circuit. Could be the bulb contacts; could be the relay; could be the toggle switch controlling the fixture. Heck, it could even be the wiring leading to that switch/fixture (tho' that would be a stretch).

In any event, I agree that pulling/replacing the bulb would be a good first step in the diagnosis process.

Thx for the chemistry lesson.

Thank you.

 

[Kevin C]

You're welcome. But see below.



I missed the "It happened suddenly" part first time through. That does indeed imply a bad/marginal connection SOMEWHERE in the circuit. Could be the bulb contacts; could be the relay; could be the toggle switch controlling the fixture. Heck, it could even be the wiring leading to that switch/fixture (tho' that would be a stretch).

In any event, I agree that pulling/replacing the bulb would be a good first step in the diagnosis process.



Thank you.

I did find it kind of funny that you missed that.... Esp considering the length of your answer!!!!

That type of lamp seems to have a lot of issues with the contacts... very common problem.

BTW nice write up on bulbs. One thing that has changed is that most non halogen bulbs do not have a vacuum, lower than atmospheric pressure but they just use a lower cost inert (typically argon / nitrogen) gas.

Some small bulbs use a vacuum.